The present invention relates generally to gaskets, and more particularly, to an improved spiral-wound gasket having an outer ring and an inner ring. The present gasket is adapted to be disposed between flanges of a pipe or vessel to prevent fluid leakage. Sections of pipe are connected to form a pipeline to direct various fluids from one location to another. To facilitate connection of the pipes, flanges are formed on the ends which may be connected to a subsequent pipe flange using a plurality of bolts. The gasket is inserted between the flanges to prevent fluid leakage.
The evolution of a new gasket type for use in Hydrofluoric (HF) Alkylation Units from the standard HF alkylation spiral-wound type gaskets, (monel windings, PTFE filler, and outer carbon steel ring) to a more specialized and robust gasket type was driven by a need to minimize flange face corrosion, overcome handling limitations and improve sealing performance. One design objective was to protect the carbon steel flange face from aggressive HF acid corrosion and resulting iron fluoride scaling, while increasing both the reliability and sealability of an HF connection. The desired result was to prevent costly flange damage, potential leakage and associated unit shutdowns required for repairs.
Flange corrosion due to the aggressiveness of hydrofluoric acid has been prevalent in chemical units for many years, causing thousands of dollars in maintenance, repair and replacement costs. The standard ASME B16.20 spiral-wound type gasket with monel windings, PTFE filler and an outer carbon steel locating ring left the flange face within the inside diameter of the gasket vulnerable to exposure from the corrosive process.
To prevent corrosion along the entire flange face, the gasket would need to begin sealing the process at the bore, preventing the acid from migrating outward and contaminating the mating flanges surfaces (see FIG. 1).
The first attempt to protect the flange faces from the corrosive hydrofluoric acid was the specification of a standard inner ring. In 1993, ASME B16.20 mandated the use of inner rings on all standard spiral-wound type gaskets with the PTFE filler material to prevent inward buckling of the spiral windings. Since the initial gasket design lacked an inner ring, the new specification included a monel inner ring. However, even with the inner ring, when the flanges were opened for routine maintenance, flange face corrosion was prevalent from the bore to the inside diameter of the raised face.
It was evident that the inclusion of an inner ring alone was not the solution. Although the inner ring of a spiral-wound type gasket reinforces the inside diameter of the winding element and prevents buckling, it unfortunately does not function as a direct sealing element. In fact, it was discovered that this configuration allowed the corrosive process to seep under the inner ring and cause even more extensive damage to the flange faces than without an inner ring, (see FIG. 2).
The need for a gasket to not only seal the process, but also prevent corrosion of the inside diameter of the mating flange faces was now imperative. To accomplish this, the gasket would need to seal the process at the bore, not just on the raised face. This meant the inner ring would need to effectively seal.
The second attempt to protect the flange faces was the specification of a PTFE inner ring within the standard spiral-wound type gasket. A 0.150″ thick sintered PTFE ring was cut to meet the standard inner ring dimensions per ASME B16.20 and inserted into the standard ASME B16.20 spiral-wound type gasket with monel windings, PTFE filler and an outer carbon steel locating ring. The 0.150″ thickness of the PTFE inner ring allowed for compressive load to be applied and promote sealing on the inner ring portion of the gasket without affecting the compressibility of the spiral windings. Unfortunately, due to a high coefficient of thermal expansion, the PTFE inner ring was notorious for shrinking and dislodging from the spiral windings, especially during handling, (see FIG. 3).
It became evident that a PTFE sheet gasket material was not a durable solution for achieving a seal as an inner ring. The decision was made to return to a metal inner ring. However, the standard configuration of a spiral-wound type gasket with a costly monel inner ring had already proved to be ineffective against preventing corrosion over the entire flange face.
Drawing from the success of another gasket type in the industry, the standard 0.125″ thick inner ring was added to the standard spiral-wound type gasket with monel windings, PTFE filler and a carbon steel locating ring, and modified to include machined serrations on the ring faces, as seen in the “kammprofile”, or serrated metal type gasket. The term “kammprofile” refers to a “comb profile” and is used to describe a gasket design or a portion of a gasket, such as an inner ring, having serrations on each face. As used herein, this term is defined to include such a serrated construction that is coated in PTFE and faced with either graphite or PTFE. Carbon steel was chosen as the inner ring metal and a U-shaped PTFE envelope was incorporated to protect the inside diameter and faces of the carbon steel inner ring. This modified spiral-wound type gasket now contained “dual” sealing components: the kammprofile type inner ring and the spiral winding elements which seal on the raised face, (see FIG. 4).
When the flanged connections were opened after utilizing this type of gasket, it was found that in some cases the PTFE enveloped hindered successful sealing of the inner ring due to various factors: 1) envelopes were prone to folding, perhaps during installation, leaving areas of the serrated metal exposed and allowing corrosion to take place; 2) shearing of the seams of the U-shaped envelope and extrusion of the PTFE into the bore was observed in some of the flanges where corrosion had taken place; and 3) in other cases, PTFE envelopes were found downstream, completely dislodged from the gasket serrated inner ring.
The inner ring design was again modified. The standard dimensions of an inner ring per ASME B16.20 were maintained and kammprofile type serrations were machined in the faces of each ring as before. However, this time the serrated inner ring was coated with PTFE to prevent direct contact of the process with the metal. The PTFE coated machined serrations were then faced with 0.020″ thick flexible graphite. The monel winding metal was maintained, however the filler material was changed from PTFE to flexible graphite the provide a “fire-safe” feature, (see FIG. 5).
Although the foregoing gasket evolution resulted in a construction that addressed several of the identified performance issues, there remained an opportunity for still further gasket design improvements. The focus of these further improvements, as disclosed herein, is directed to the construction and arrangement of the spiral-wound portion of the gasket and its combination with the remainder of the gasket construction.